Legal regulations have now been imposed with respect to the problem of air pollution, and this problem is of great importance all over the world. There are many sources of this air pollution, and one of them is the exhaust gas from engines. This gas contains carbon dioxide and water vapor, which are the products of complete combustion, and nitrogen, which originates from air introduced into the combustion chamber. It is known that combustion in an engine using a hydrocarbon fuel is an incomplete combustion, regardless of whether the engine is of the internal combustion type or of another type. For instance, exhaust gas coming from an automobile engine contains components regarded as direct or indirect poisons, such as carbon monoxide, unburnt hydrocarbons, nitrogen oxides and the like.
Carbon monoxide is poisonous and colorless and is substantially odorless. Accordingly, it can hardly be detected by the human body. Hydrocarbons per se are less poisonous, but they react with components contained in air and bring about substantial air pollution.
Therefore, since the appearance of automobiles, various efforts have been made to render the exhaust gas from an engine non-toxic. One of these is a method using a catalyst device for purifying exhaust gases. Catalysts to be used for such a device encounter the following problems. Firstly, exhaust gas from an engine varies greatly as to the harmful substances which it contains, the exhaust gas temperature, the flow rate and the like, depending on the engine operating conditions, and it is difficult to establish typical exhaust gas conditions; therefore, the catalysts should have a sufficient activity under a broad range of conditions.
Secondly, since the combustion temperature in an engine rises above 800.degree. C., the catalysts to be used are required to be such that any reduction in their activity at high temperatures is kept low.
When a catalyst device of this type is used for automobiles, the conversion of harmful substances from the engine to harmless substances should be effected before the exhaust gas is discharged into the open air from an automobile. Therefore, catalysts should have sufficient strength to resist break-up when subjected to vibration during running.
Compounds of lead, sulfur, phosphorus and the like are included in fuels for automobile internal combustion engines, and hence, compounds of lead, sulfur and phosphorus exist in an exhaust gas as a matter of course. Furthermore, phosphorus compounds are contained in lubricating oils for internal combustion engines. Since a lubricating oil is often burnt in an internal combustion engine, components originating from the lubricating oil are contained in the exhaust gas. Among these compounds in exhaust gases, the compounds of lead, sulfur and phosphorus are generally harmful to the catalysts. Therefore, the catalysts should have a sufficient resistance to these harmful substances, which are known as "catalyst poisons".
In general, the mechanical resistance and resistance to catalyst poisons in exhaust gases are greatly influenced by the carriers used for the catalyst for purifying exhaust gases, and the heat resistance of the catalyst is greatly influenced not only by the carrier but also the active catalytic component, namely the catalytic metal component.
However, catalysts for purifying exhaust gases which have a sufficient mechanical strength and a sufficient resistance to catalyst poisons have not been easily found.